Polarisation-dependent single-pulse ultrafast optical switching of an elementary ferromagnet

The ultimate control of magnetic states of matter at femtosecond (or even faster) timescales defines one of the most pursued paradigm shifts for future information technology. In this context, ultrafast laser pulses developed into extremely valuable stimuli for the all-optical magnetization reversal in ferrimagnetic and ferromagnetic alloys and multilayers, while this remains elusive in elementary ferromagnets. Here we demonstrate that a single laser pulse with sub-picosecond duration can lead to the reversal of the magnetization of bulk nickel, in tandem with the expected demagnetization. As revealed by realistic time-dependent electronic structure simulations, the central mechanism involves ultrafast light-induced torques that act on the magnetization. They are only effective if the laser pulse is circularly polarized on a plane that contains the initial orientation of the magnetization. We map the laser pulse parameter space enabling the magnetization switching and unveil rich intra-atomic orbital-dependent magnetization dynamics featuring transient inter-orbital non-collinear states. Our findings open further perspectives for the efficient implementation of optically-based spintronic devices.

The Influence of the Processing Parameters on the Laser-Ablation of Stainless Steel and Brass during the Engraving by Nanosecond Fiber Laser

In this paper, we investigate the influence of the following parameters: pulse duration, pulse repetition rate, line-to-line and pulse-to-pulse overlaps, and scanning strategy on the ablation of AISI 316L steel and CuZn37 brass with a nanosecond, 1064-nm, Yb fiber laser. The results show that the material removal rate (MRR) increases monotonically with pulse duration up to the characteristic repetition rate (f0) where pulse energy and average power are maximal. The maximum MRR is reached at a repetition rate that is equal or slightly higher as f0. The exact value depends on the correlation between the fluence of the laser pulses and the pulse repetition rate, as well as on the material properties of the sample. The results show that shielding of the laser beam by plasma and ejected material plays an important role in reducing the MRR. The surface roughness is mainly influenced by the line-to-line and the pulse-to-pulse overlaps, where larger overlap leads to lower roughness. Process optimization indicates that while operating with laser processing parameters resulting in the highest MRR, the best ratio between the MRR and surface roughness appears at ~50% overlap of the laser pulses, regardless of the material being processed.